1. Field of the Invention
The invention relates generally to improvements in concrete panels used as exterior cladding for buildings. More specifically, the invention pertains to a composite panel, comprising a pre-stressed concrete slab structurally integrated with a support frame.
2. Description of the Prior Art
One method of constructing modern buildings employs a structural steel or cast-in place concrete supporting framework, upon which all other building elements are mounted and supported. These building elements, which are integrated with and attached to the steel or concrete supporting framework, include walls, floors, and electrical, plumbing, and HVAC systems.
The exterior of the supporting framework is covered with panels or cladding, approximately 5 to 15 feet in height, and 15 to 35 feet in length, and cementitious in nature. The panels are generally in the range of 4 to 5 inches thick, and weigh approximately 50 to 65 pounds per square foot. Typically, the cladding is cast off-site, and then transported to the construction site after the supporting framework of the building is completed. Using cranes or hoists, the cladding is lifted into place and attached to an interior building framework using fasteners. Aspects of such construction are shown and explained in U.S. Pat. No. 6,823,633, for a Secondary Moisture Drainage System For Structures Having Pre-Manufactured Exterior Cladding Systems. U.S. Pat. No. 6,823,633 is hereby specifically incorporated by reference into the present disclosure.
Owing to their durability and architectural appeal, pre-manufactured concrete cladding systems have been used extensively for exteriors of commercial buildings. In this capacity, cementitious cladding is subjected to degradation from natural forces such as wide temperature variations, moisture intrusion, ultra-violet rays, wind loading, and seismic loading. Man-made forces such as vibrations from traffic, construction within the building, and demolition and new construction in the vicinity of the building, may all contribute to weakening of the cladding over time. As a consequence, prior art cladding systems are manufactured as “heavy elements” of the overall building structure, imposing high loads on the supporting framework.
Buildings designed to carry both the lateral and the gravity loads imposed by such heavy concrete cladding systems must include additional reinforcement and upgrading of structural members, and are therefore more costly than other construction methods. It should also be noted that prior art concrete cladding systems typically require secondary interior framing to attach interior insulation and finishes. This secondary interior framing requires additional materials and labor, and also adds to the overall construction costs of the building.
It would therefore be desirable if the weight of the cladding could be reduced significantly, while still providing the inherent functional and aesthetic advantages of the cladding system. Such a reduction in weight would allow use of a supporting framework that would be less costly to design and implement. It would also be desirable if the necessity of secondary interior framework of the prior art cladding systems could be eliminated, saving again both labor and materials.
Pre-stressing concrete has long been recognized as a technique to increase the tensile strength of cast concrete structures. Through the use of a pre-stressing technique in the casting of concrete, it is possible to make a given structure stronger than a corresponding concrete structure which employs more conventional reinforcement means, such as mesh, rods, and the like. The pre-stressing technique may be used advantageously to increase the strength of poles, beams, slabs, and panels, for example.
The pre-stressing technique generally requires that high strength wires, cables, or rods, passing through the empty mold or form for the concrete structure, are pre-stressed under high tension using a calibrated tensioning fixture. Then, the concrete is poured into the mold or form, enveloping the pre-stressed wires or cables. After the concrete has sufficiently cured, the ends of the wires or cable extending outside the mold are cut from the tensioning fixture, transferring the compressive forces to the concrete through the bond between the wires or cables and the concrete.
The general principles of this technique are illustrated in U.S. Pat. No. 6,773,650, issued to Longo for a Prestressed Concrete Casting Apparatus And Method. The '650 patent illustrates a pre-stressing clamshell device designed to cast cementitious power poles. In this arrangement, a plurality of stationary, cable pre-tensioning devices are lined up at a production facility. The movable clamshell mold surrounds each pre-tensioning fixture while the concrete is poured and allowed to set. Then, the mold is opened and lifted up, and then moved along to the adjacent fixture, where the process is repeated.
In Patent Application Publication US 2006/0230706, owned by Skendzie et al., a disclosure is made of Constructing The Large-Span Self-Braced Buildings Of Composite Load-Bearing Wall-Panels And Floors. Galvanized steel sheet strips 4 are used to interconnect two concrete panels 1 in both wall and floor applications (See, FIG. 7). Two steel wire mesh layers 5 are used in conjunction with reinforcing bars 6, between the mesh layers, to reinforce the panels 1 (See, FIG. 1). In Paragraph [38] of this publication, it is stated that the reinforcing bars 6 can be replaced by pre-stressing wire-strands (not shown), depending upon the desired degree of pre-stressing. FIGS. 9 and 10 show the casting form used for manufacturing panels 1.
The general concept of interconnecting a cementitious panel to studs, through connectors embedded in a concrete panel and extending outside the panel, is also shown in the prior art. Pre-fabricated Building Panels And Method Of Manufacturing are disclosed in U.S. Pat. No. 6,729,094, granted to Spencer et al. The concrete panel 10 includes a concrete slab 11 with a metal mesh 16 embedded in the slab. An insulating panel 12 is contingent upon a surface of the concrete slab 11. One or more studs 13, having top and bottom edges secured within upper and lower U-shaped tracks 54, are interconnected to one or more connectors 17 embedded in the slab 11.